1. Field of the Invention
The present invention relates generally to methods and apparatus for electrically isolating a rail on a concrete grade crossing and more particularly to such a method and apparatus utilized in connection with a grade crossing formed of precast concrete as opposed to concrete cast in place.
2. Description of the Related Art
Special preparation of a rail bed is necessary whenever a road or pedestrian walkway crosses a rail bed at grade level. Generally such crossings are constructed such that the surface of the road is at approximately the same level as the upper surface of the rails. The recessed area between the rails and to either side thereof are built up to street level with the exception that recesses must be provided adjacent the upper portion of each rail to accommodate the flanges on railcar wheels.
This crossing construction has been accomplished in the past by filling the recessed area with materials such as wood or asphaltic paving materials. Cast-in-place and precast concrete crossing structures have also been used.
Elastomeric boots have been utilized in the past in connection with precast concrete grade crossings as well as with cast-in-place concrete grade crossings which are formed at the site of the crossing by pouring concrete into a form containing the boot-encased rail. Boots may be made of a rubbery compound such as synthetic butyl rubber or EPDM and may be a 3/16 to 1/2 inch in thickness. The material used in the past has a Durometer of 80-85. Other such prior art boots have a thickness of 0.165 or 0.200 inch with a Durometer of 70 .+-.5.
One prior art boot used in connection with a precast concrete grade crossing system is disclosed in U.S. Pat. No. 4,641,779 to O'Brien, et al. In this system, the boot entirely encases the flange or lower portion of each rail. The boot extends from the flange along each side of the rail and terminates, on both sides of the rail, at a point just beneath the head or upper portion of the rail upon which a train wheel is supported. While the boot disclosed in the O'Brien, et al. patent works well to cushion the rail against abrasion and vibration, because the entire head of the rail is exposed, there may be electrical conductivity between the rail and the precast concrete in which it is secured. This condition is aggravated in regions where salt is applied to roads in snowy and icy weather by increasing the conductivity of water accumulating on the crossing. When the rail is not electrically isolated, it cannot be effectively used as a signal conductor. In addition, current flowing between the rail and the concrete accelerates corrosion.
In the O'Brien, et al. system, isolation of the gauge or inner side of each rail has been achieved by installing an elastomeric flange in the flangeway adjacent the gauge side of each rail. Such a flange is depicted immediately to the right of the sectional view in FIG. 2 herein. Even with the flange, however, isolation of this prior art system was not totally effective for two reasons. First, the boot on the field or outer side of the rail terminated beneath the head of the rail thereby exposing the outer portion of the head to the elements. Secondly, the boots in this prior art system were not installed in continuous lengths throughout the crossing. They were, rather, installed in sections with seams between each section presenting the potential for leakage currents flowing between the rail and the concrete.
One prior art boot has been proposed for use in connection with a grade crossing of the type which is cast in place. The lower portion of this prior art boot encases the base and web of the rail as is the case with the O'Brien, et at. patent. The gauge side of the prior art boot extends upwardly to cover substantially all of the underside of the head. The field side of the prior art boot extends vertically up the field side of the head. Thereafter, the rail is positioned in the crossing and concrete is placed so as to surround that portion of the rail encased by the boot.
It would be desirable to provide an elastomeric boot for a rail in a precast concrete grade crossing which covers the field side of the head and which is held in place so as not to permit water and other materials from leaking into the space between the boot and the rail.
It would also be desirable to eliminate seams between adjacent lengths of elastomeric boot in precast concrete grade crossings.